The invention relates to an electrical protection device comprising a plurality of sensors, a multiplexer connected to the output of the sensors to receive a plurality of electrical signals to be measured with a predetermined precision, an analog-to-digital converter, connected to the output of the multiplexer, and digital processing means connected to the analog-to-digital converter.
It is standard usage, in electrical protection devices, and more particularly in an electronic trip device of an electrical circuit breaker, to multiplex the currents and/or voltages of an electrical power system to be protected, and then to perform an analog-to-digital conversion before processing them in a digital processing circuit, preferably microprocessor-based (U.S. Pat. Nos. 5,101,316, 4,672,501).
It has moreover been proposed to use sigma-delta type analog-to-digital converters in an electronic trip device. In this case, the trip device comprises a converter associated to each measuring channel, i.e. to each current sensor. In the document U.S. Pat. No. 4,943,888, three sigma-delta converters are required. In the case of a top-of-the-range trip device there may be up to 8 measuring channels, one for the currents of the three phases, one for the neutral current, three for the voltages of the three phases and one for the ground fault current, which would mean using 8 converters.
The use of a multiplexer in combination with a sigma-delta type analog-to-digital converter has not been envisaged up to now in a protection device due to the problems arising from the transient signals able to be created by a sudden front when switching from one measuring channel to the next.
The object of the invention is to improve the measuring precision of an electrical protection device, more particularly of an electronic trip device, with a reduced cost and volume.
According to the invention, this object is achieved by the fact that the analog-to-digital converter is of the sigma-delta type, comprising a sigma-delta modulator, having a preset stabilization time, connected in series with a numerical decimation filter, the decimation filter comprising a plurality of weighting coefficients of level 1 to n, such that, a first quantity corresponding to the ratio between the sum of the coefficients of the decimation filter, of level 1 to m, used during the modulator stabilization time, and the sum of all the coefficients of the filter, a second quantity corresponding to the ratio between the maximum and minimum amplitudes of the electrical signals to be measured, the product of the first and second quantities is lower than said precision, m being much lower than n.
Combining a multiplexer and a sigma-delta type converter, made possible by a suitable choice of the decimation filter coefficients, enables the high resolution and excellent linearity of the sigma-delta converter to be combined with the space and cost saving resulting from the use of a multiplexer and a single analog-to-digital converter.